1. Field of the Invention
The present invention is related to an apparatus for switching the suction system in a vacuum pump, and particularly to an apparatus for switching the suction system in a vacuum pump, wherein two suction pump sections are connected in parallel at the initial start-up time to increase the suction amount and automatically switched to a series connection after a preset negative pressure has been reached to provide vacuum or a degree of reduced pressure near vacuum at a high speed or in a short time, whereby an efficient reduced or vacuum pressure can be obtained.
2. Description of the Prior Art
In the U.S. Pat. No. 4,718,832 specification, a double-pump unit is disclosed in which two pumps are united or ganged. Since the use of such pump unit allows the connection mode of suction inlet and discharge outlet of each pump to optionally be set to either series connection or parallel connection, the suction amount can be set in multiple stages when such pump unit is used as a suction pump, without causing so much loss of the energy of an a.c. power supplied to the stationary electromagnets of the pumps.
FIG. 4 is a schematic diagram showing a state in which first pump 1 and second pump 2 are connected in parallel, and FIG. 5 is a schematic diagram showing a state in which first pump 1 and second pump 2 are connected in series, wherein symbols 1A and 2A represent suction inlets and symbols 1B and 2B represent discharge outlets. If, in these figures, the pumps are assembled so that the axis of each piston of the respective pumps is positioned on a single straight line and the pistons move in the directions opposite to each other, the vibrations of the respective pumps offset each other, whereby the vibration and noise of the pump unit can be reduced.
FIG. 6 shows an example of the pump characteristics when two vacuum pumps having the same performance are connected in series and when they are connected in parallel. In the same figure, the abscissa axis is negative pressure while the ordinate axis is flow rate, and the solid line represents the series connection performance while the dotted line represents the parallel connection performance. The characteristics of a single vacuum pump is shown by the dashed line.
As shown in FIG. 6, if two pumps of the same performance are connected in series, the flow rate when the negative pressure is zero (open-to-atomosphere state) is the same as that of a single pump, but the pressure when the flow rate is zero (cut-off state) becomes lower than that of a single pump, that is, the negative pressure becomes more negative than when using the single pump. Conversely, if the two pumps are connected in parallel, the negative pressure when the flow rate is zero is same as that of the single pump, but the flow rate when the negative pressure is zero becomes approximately two times that of the single pump.
Accordingly, if, at the pressure (P1) at which the series connection performance line (solid line) and the parallel connection performance line (dotted line) are intersecting each other, the connection is switched so that the pumps are connected in parallel when the negative pressure is in the range from 0 through P1 and they are changed to a series connection when the negative pressure has become more negative than P1 (or the pressure has become lower than P1), then an extremely efficient operation is obtained.
In order to perform such operation, it is only needed to provide a pressure sensor for sensing the negative pressure P1, for instance, in the fluid passages of the pumps and operate electromagnetic valves or the like to switch the fluid passages. In the above described prior art, however, there is disclosed no means for automatically switching the suction system. If an attempt is made to add the sensor and the electromagnetic valves for switching the fluid passages to the pump unit of the above prior art and perform the parallel-series connection switching, a power supply is required to supply power to the electromagnetic valves and sensor to be added, so the construction becomes further complex.